In the pharmaceutical industry, there are a number of medicaments which are already filled into medical syringes at the place of manufacture. The pharmaceutical manufacturer fills the medicament, under sterile conditions, into a sterile syringe barrel which is closed off at the distal end with a sealing cap. The syringe barrel is then closed off at the proximal end with a sterile plunger stopper, frequently made of rubber-elastic material. Thereafter, the plunger rod of the syringe is usually screwed into a suitably provided internal thread on the plunger stopper, the syringe is placed in a sterile package, and made ready for supply. A particular advantage of this procedure is that the medicament is in this way kept sterile until actual administration, which cannot always be guaranteed if, for example, the medicament is drawn into the syringe just before administration.
EP 0 764 450 B1 discloses a syringe which is well suited for application purposes of this kind because a backstop at the proximal end of the syringe barrel prevents the plunger stopper from being pulled out rearward, i.e. in the direction of the proximal end, after the syringe has been filled and assembled. Inadvertent removal of the plunger stopper, which would compromise sterility, can be ruled out in this way. In addition, a syringe having a backstop is particularly suitable for very expensive and/or highly toxic medicaments, for example anti-cancer agents, because, if the plunger stopper were inadvertently pulled out, the medicament contained in the syringe could escape and thus pose a hazard to the medical personnel or would at least represent an expensive loss.
A further preferred application of syringes with a backstop is when the medicament to be administered has to be prepared by being drawn up several times into the syringe. In these cases too, the backstop ensures that the plunger stopper cannot be inadvertently pulled back too far and thus removed from the syringe barrel.
In the syringe of EP 0 764 450 B1, the backstop consists of a grip plate and of a flange formed integrally thereon. After insertion of the plunger stopper into the syringe barrel, the backstop is fitted laterally onto the syringe barrel, i.e. orthogonally with respect to the longitudinal direction of the syringe and thus orthogonally with respect to the direction of movement of the plunger stopper. The finger plate and the flange have a lateral aperture permitting this lateral engagement or snap-fit onto the outer circumference of the syringe barrel. Moreover, the finger plate and the flange have, at the outer proximal end of the syringe barrel, an internal diameter which is smaller than the external diameter of the plunger stopper, by which means the backstop is formed.
WO 99/55402 discloses a syringe in which the backstop is formed by a projection arranged in the inside of the syringe barrel. Specially designed locking hooks are provided on the plunger rod of the syringe and these can snap over the projection in the direction of advance of the plunger rod, whereas they block it in the opposite direction.
WO 94/26334 discloses a prior art syringe in which the backstop is a flange which is fitted from the side at the proximal end of the syringe barrel. The flange has a bracket which protrudes into the inside of the syringe barrel and which prevents withdrawal of the plunger stopper.
In addition, there are a great many other alternatives in which a backstop is used to prevent inadvertent rearward withdrawal of the plunger stopper of a syringe. Reference is made, by way of example, to WO 94/13339, WO 00/07648, JP 2001-327600A, U.S. Pat. No. 5,250,030, DE 44 34 644 C2 or EP 0 738 517 B1. However, compared to syringes without a backstop, many of the known solutions require changes and/or additions to be made to the syringe barrel and/or to the plunger rod. This means that “conventional” syringes cannot be equipped with a corresponding backstop. As a result of this, these “special makes” are considerably more expensive than conventional syringes without a backstop.
The backstop known from aforementioned EP 0 764 450 B1 can, by contrast, be used on syringe barrels of standard design, in particular on ones with a circular flange. However, the backstop is fitted in a separate assembly step and, additionally, in a direction of movement which is transverse with respect to the direction of movement in which the plunger stopper is inserted into the syringe barrel. The additional assembly step, also deviating from the other movements, entails added costs going beyond the simple material costs for the backstop.